I think WAAS/EGNOS is really for creating a synthetic glideslope which you fly like an ILS. That's ultra slick, if you have the latest kit, but is quite some years away anywhere in Europe, IMHO. They have it in the USA, of course.
Yes, it will give you a 3D accuracy of better than 30 feet I believe. Does it need to be supported by an adequate RAIM prediction - that I dont know, but I guess it would?
I don't know the IAP design criteria (PAN/OPS or TERPS) but a GPS IAP should be no worse than a VOR/DME one.
Yes, I would also have thought so, so realistically the best one could hope for is that airports with only an NDB DME (eg EGKA) with a PGPS the MDH could be expected to fall in line with a VOR DME procedure. However it is interesting how many pilots would not accept an approach down to and with a cloud break at minima on the 20 approach even though legal because of the proximity of the close ground. Has this got more to do with the concerns over sheer and turbulence close to high ground on the approach or the implied greater inaccuracy of the present NDB DME procedure and how much more comfortable would pilots be with either a GPS or localiser procedure so far as the MDH is concernded.
However it does mean at airports with less demanding terrain on the approach presumably an MDH the same as for a VOR DME procedure at the same airport is theoretically equally as safe. Is that in fact what we will see or do the procedure writers have a hang up that the horizontal reliability of GPS is more suspect than a VOR?